阅读说明：本技术 三相半桥-串联绕组拓扑结构切换的逆变器及其切换方法 (Inverter switched by three-phase half-bridge-series winding topological structure and switching method thereof ) 是由 蒋栋 李安 刘自程 孙翔文 于 2019-09-12 设计创作，主要内容包括：本发明公开了一种三相半桥-串联绕组拓扑结构切换的逆变器及其切换方法,逆变器包括第一桥臂、第二桥臂、第三桥臂、第四桥臂以及第一双向晶闸管、第二双向晶闸管、第三双向晶闸管、第四双向晶闸管,驱动四个双向晶闸管的导通状态,可以切换逆变器的三种模式,分别对应三相半桥拓扑结构、暂态结构和串联绕组拓扑结构。本发明在模式切换时利用了两相电流的过零点进行模式切换,使切换过程短暂迅速,在切换过程中不对电机的转速转矩造成影响,从而避免了对用户的影响,同时保证逆变器低速时的转矩输出能力和高速时的转速输出能力,实现电机工作区间的最大化。(The invention discloses an inverter for switching a three-phase half-bridge-series winding topological structure and a switching method thereof. The invention utilizes the zero crossing point of the two-phase current to carry out mode switching during mode switching, so that the switching process is short and rapid, and the rotating speed and the torque of the motor are not influenced during the switching process, thereby avoiding the influence on users, simultaneously ensuring the torque output capacity of the inverter at low speed and the rotating speed output capacity of the inverter at high speed, and realizing the maximization of the working interval of the motor.)

1. The inverter is characterized by comprising a first bridge arm, a second bridge arm, a third bridge arm, a fourth bridge arm, a first bidirectional thyristor, a second bidirectional thyristor, a third bidirectional thyristor and a fourth bidirectional thyristor; each bridge arm comprises an upper bridge arm power switch device and a lower bridge arm power switch device, an upper node of the upper bridge arm power switch device of each bridge arm is connected with a direct current bus voltage, a lower node of the lower bridge arm power switch device is connected with a power ground, and a lower node of the upper bridge arm power switch device is connected with an upper node of the lower bridge arm power switch device and serves as an output node of the bridge arm;

the left node of the A-phase winding is connected with the output node of the first bridge arm, and the right node of the A-phase winding is connected with the right node of the first bidirectional thyristor and the left node of the second bidirectional thyristor;

the left node of the phase B winding is connected with the output node of the second bridge arm, and the right node of the phase B winding is connected with the right node of the third bidirectional thyristor and the left node of the fourth bidirectional thyristor;

the left node of the C-phase winding is connected with the output node of the third bridge arm, and the right node of the C-phase winding is connected with the output node of the fourth bridge arm;

the left node of the first bidirectional thyristor is connected with the output node of the second bridge arm, the left node of the third bidirectional thyristor is connected with the output node of the third bridge arm, and the right nodes of the second bidirectional thyristor and the fourth bidirectional thyristor are both connected with the output node of the fourth bridge arm.

2. The inverter of claim 1, wherein the four triacs are used to switch the inverter topology:

when the second bidirectional thyristor and the fourth bidirectional thyristor are conducted and the first bidirectional thyristor and the third bidirectional thyristor are turned off, the three-phase half-bridge topological structure is formed;

when the first bidirectional thyristor and the third bidirectional thyristor are conducted and the second bidirectional thyristor and the fourth bidirectional thyristor are turned off, the structure is a series winding topological structure;

when the first bidirectional thyristor and the fourth bidirectional thyristor are conducted and the second bidirectional thyristor and the third bidirectional thyristor are turned off, the structure is a transient structure.

3. The inverter of claim 1, wherein the power switching devices are fully current controlled switches, MOSFETs or IGBTs with anti-parallel diodes.

4. A topology switching method for a three-phase half-bridge-series winding topology switched inverter, comprising:

driving a second bidirectional thyristor and a fourth bidirectional thyristor to be conducted, turning off the first bidirectional thyristor and the third bidirectional thyristor, removing a driving signal of the second bidirectional thyristor when receiving an instruction of switching from a first mode to a second mode, turning off the second bidirectional thyristor after waiting for the natural zero crossing of the phase-A current, driving the first bidirectional thyristor to be conducted, and simultaneously removing a driving signal of the fourth bidirectional thyristor, switching the mode to a transient mode, turning off the fourth bidirectional thyristor after waiting for the natural zero crossing of the phase-B current, driving the third bidirectional thyristor to be conducted, and switching the mode to the second mode;

and driving the first bidirectional thyristor and the third bidirectional thyristor to be conducted, turning off the second bidirectional thyristor and the fourth bidirectional thyristor, removing a driving signal of the third bidirectional thyristor when receiving an instruction for switching from the second mode to the first mode, turning off the third bidirectional thyristor after waiting for the natural zero crossing of the phase B current, driving the fourth bidirectional thyristor to be conducted, simultaneously removing the driving signal of the first bidirectional thyristor, switching the mode to the transient mode, turning off the first bidirectional thyristor after waiting for the natural zero crossing of the phase A current, driving the second bidirectional thyristor to be conducted, and switching the mode to the first mode.

5. The switching method according to claim 4, wherein in the first mode, the inverter is in a three-phase half-bridge topology, in the second mode, the inverter is in a series winding topology, and in the transient mode, the inverter is in a transient configuration.

6. The switching method according to claim 4, wherein in the first mode, the current flowing into each leg can be expressed as a stator current of a three-phase alternating current motor as:

wherein i₁、i₂、i₃And i₄I represents currents flowing in the first arm, the second arm, the third arm, and the fourth arm, respectively_a、i_bAnd i_cRespectively represent the currents of the A-phase stator winding, the B-phase stator winding and the C-phase stator winding, I_acIs the effective value of the AC component of the stator winding current, theta_eIs an electrical angle and is related to the rotor angle.

7. The switching method according to claim 4, wherein in the second mode, the current flowing into each leg can be expressed as a stator current of a three-phase alternating current motor as:

wherein i₁、i₂、i₃And i₄I represents currents flowing in the first arm, the second arm, the third arm, and the fourth arm, respectively_a、i_bAnd i_cRespectively represent the currents of the A-phase stator winding, the B-phase stator winding and the C-phase stator winding, I_acIs the effective value of the AC component of the stator winding current, theta_eIs an electrical angle and is related to the rotor angle.

8. The switching method according to claim 4, wherein in the transient mode, the current flowing into each leg can be expressed as a stator current of a three-phase alternating current motor as:

wherein i₁、i₂、i₃And i₄I represents currents flowing in the first arm, the second arm, the third arm, and the fourth arm, respectively_a、i_bAnd i_cRespectively represent the currents of the A-phase stator winding, the B-phase stator winding and the C-phase stator winding, I_acIs the effective value of the AC component of the stator winding current, theta_eIs an electrical angle and is related to the rotor angle.

Technical Field

The invention belongs to the field of alternating current motors and drive control, and particularly relates to an inverter for switching a three-phase half-bridge-series winding topological structure and a switching method thereof.

Background

The use of inverters to control ac motors is the primary method of modern electric drives. The most widely used inverter topology at present is the three-phase half-bridge topology. The topology adopts load Y-shaped connection, only comprises three bridge arms, and has low cost, small volume and high efficiency. However, under the Pulse Width Modulation (PWM) driving, the peak value of the output line voltage of the inverter cannot exceed the dc bus voltage, i.e. the phase voltage of the motor can only reach 57% of the dc voltage, which greatly limits the rotating speed output capability of the inverter at high speed of the motor. The H-bridge inverter adopting the three-phase open winding can realize the direct-current voltage utilization rate of which the phase voltage reaches 100 percent, but the switching devices and the corresponding auxiliary elements are doubled.

The Chinese invention patent 'an open winding motor driver topology and a modulation method thereof' (application number: CN201810051626.3, application date: 2018.01.19) discloses a three-phase series winding topology structure. By adding a bridge arm and changing the connection mode of the phase winding, the direct-current voltage utilization rate of the topology is twice of that of a three-phase half-bridge topology, so that the inverter has a wide speed regulation interval. However, the topological connection mode increases the current stress of the bridge arm, increases the power loss, and limits the torque output capability of the inverter at low speed of the motor.

Therefore, it is difficult for the current inverter for the motor controller to simultaneously secure the torque output capability and the rotational speed output range, and the operation range of the motor cannot be maximally utilized.

Disclosure of Invention

Aiming at the defects of the prior art, the invention aims to realize the switching of two topologies under the condition of not influencing the work of the motor by combining a three-phase half-bridge topology structure and a three-phase series winding topology structure, thereby simultaneously ensuring the torque output capacity of the inverter at low speed and the rotating speed output capacity of the inverter at high speed and realizing the maximization of the working range of the motor.

To achieve the above object, according to an aspect of the present invention, there is provided an inverter with switching three-phase half-bridge-series winding topology, including a first bridge arm, a second bridge arm, a third bridge arm, a fourth bridge arm, a first triac, a second triac, a third triac, and a fourth triac; each bridge arm comprises an upper bridge arm power switch device and a lower bridge arm power switch device, an upper node of the upper bridge arm power switch device of each bridge arm is connected with a direct current bus voltage, a lower node of the lower bridge arm power switch device is connected with a power ground, and a lower node of the upper bridge arm power switch device is connected with an upper node of the lower bridge arm power switch device and serves as an output node of the bridge arm;

the left node of the A-phase winding is connected with the output node of the first bridge arm, and the right node of the A-phase winding is connected with the right node of the first bidirectional thyristor and the left node of the second bidirectional thyristor;

the left node of the phase B winding is connected with the output node of the second bridge arm, and the right node of the phase B winding is connected with the right node of the third bidirectional thyristor and the left node of the fourth bidirectional thyristor;

the left node of the C-phase winding is connected with the output node of the third bridge arm, and the right node of the C-phase winding is connected with the output node of the fourth bridge arm;

the left node of the first bidirectional thyristor is connected with the output node of the second bridge arm, the left node of the third bidirectional thyristor is connected with the output node of the third bridge arm, and the right nodes of the second bidirectional thyristor and the fourth bidirectional thyristor are both connected with the output node of the fourth bridge arm.

Further, four triacs are used to switch the topology of the inverter:

when the second bidirectional thyristor and the fourth bidirectional thyristor are switched on and the first bidirectional thyristor and the third bidirectional thyristor are switched off, the three-phase half-bridge topological structure can provide large current but cannot provide high direct-current voltage utilization rate, so that the three-phase half-bridge topological structure is suitable for the motor to operate under the working condition of low speed and high torque;

when the first bidirectional thyristor and the third bidirectional thyristor are conducted and the second bidirectional thyristor and the fourth bidirectional thyristor are turned off, the series winding topological structure can provide high direct-current voltage utilization rate but cannot provide large current, so that the motor is suitable for running under a high-speed working condition, but the torque needs derating running;

when the first bidirectional thyristor and the fourth bidirectional thyristor are conducted and the second bidirectional thyristor and the third bidirectional thyristor are turned off, the structure is a transient structure.

Preferably, the power switch device is a current fully controlled switch, such as a MOSFET or an IGBT with an anti-parallel diode.

According to another aspect of the present invention, a topology switching method for an inverter for switching the three-phase half-bridge-series winding topology is provided, which aims to smoothly and arbitrarily switch the two topologies, so that the switching process is as short and fast as possible, and the rotational speed and torque of the motor are not affected during the switching process, thereby avoiding affecting the user experience. The control method comprises the following steps:

driving a second bidirectional thyristor and a fourth bidirectional thyristor to be conducted, turning off the first bidirectional thyristor and the third bidirectional thyristor, removing a driving signal of the second bidirectional thyristor when receiving an instruction of switching from a first mode to a second mode, turning off the second bidirectional thyristor after waiting for the natural zero crossing of the phase-A current, driving the first bidirectional thyristor to be conducted, and simultaneously removing a driving signal of the fourth bidirectional thyristor, switching the mode to a transient mode, turning off the fourth bidirectional thyristor after waiting for the natural zero crossing of the phase-B current, driving the third bidirectional thyristor to be conducted, and switching the mode to the second mode;

and driving the first bidirectional thyristor and the third bidirectional thyristor to be conducted, turning off the second bidirectional thyristor and the fourth bidirectional thyristor, removing a driving signal of the third bidirectional thyristor when receiving an instruction for switching from the second mode to the first mode, turning off the third bidirectional thyristor after waiting for the natural zero crossing of the phase B current, driving the fourth bidirectional thyristor to be conducted, simultaneously removing the driving signal of the first bidirectional thyristor, switching the mode to the transient mode, turning off the first bidirectional thyristor after waiting for the natural zero crossing of the phase A current, driving the second bidirectional thyristor to be conducted, and switching the mode to the first mode.

Further, in the first mode, the inverter is in a three-phase half-bridge topology structure, the second bidirectional thyristor and the fourth bidirectional thyristor are switched on, and the first bidirectional thyristor and the third bidirectional thyristor are switched off; in a second mode, the inverter is in a series winding topological structure, the first bidirectional thyristor and the third bidirectional thyristor are conducted, and the second bidirectional thyristor and the fourth bidirectional thyristor are turned off; in the transient mode, the inverter is in a transient structure, the first bidirectional thyristor and the fourth bidirectional thyristor are conducted, and the second bidirectional thyristor and the third bidirectional thyristor are turned off.

Further, in the first mode, the current flowing into each arm can be expressed by the stator current of the three-phase alternating-current motor as:

wherein i₁、i₂、i₃And i₄I represents currents flowing in the first arm, the second arm, the third arm, and the fourth arm, respectively_a、i_bAnd i_cRespectively represent the currents of the A-phase stator winding, the B-phase stator winding and the C-phase stator winding, I_acIs the effective value of the AC component of the stator winding current, theta_eIs an electrical angle and is related to the rotor angle.

Further, in the second mode, the current flowing into each arm can be expressed by the stator current of the three-phase alternating-current motor as:

wherein i₁、i₂、i₃And i₄I represents currents flowing in the first arm, the second arm, the third arm, and the fourth arm, respectively_a、i_bAnd i_cRespectively represent the currents of the A-phase stator winding, the B-phase stator winding and the C-phase stator winding, I_acFor alternating current in stator winding currentEffective value of minutes, θ_eIs an electrical angle and is related to the rotor angle.

Further, in the transient mode, the current flowing into each arm can be expressed as the stator current of the three-phase ac motor as:

wherein i₁、i₂、i₃And i₄I represents currents flowing in the first arm, the second arm, the third arm, and the fourth arm, respectively_a、i_bAnd i_cRespectively represent the currents of the A-phase stator winding, the B-phase stator winding and the C-phase stator winding, I_acIs the effective value of the AC component of the stator winding current, theta_eIs an electrical angle and is related to the rotor angle.

Through the technical scheme, compared with the prior art, the invention has the following beneficial effects:

1. the inverter switched by the three-phase half-bridge-series winding topological structure utilizes the zero crossing point of two-phase current to carry out mode switching during mode switching, so that the switching process is short and rapid, and the rotating speed torque of a motor is not influenced during the switching process, thereby avoiding the influence on a user, simultaneously ensuring the torque output capacity of the inverter at low speed and the rotating speed output capacity of the inverter at high speed, and realizing the maximization of the working interval of the motor by utilizing the respective advantages of the two topologies;

2. compared with a three-phase half-bridge topology, the inverter provided by the invention can expand the rotating speed range nearly doubled under the condition of no weak magnetism, and has all current control freedom and better fault-tolerant performance;

3. compared with a three-phase full-bridge topology, the inverter provided by the invention has the advantages that the number of switching devices required in the same working range is less, the trigger circuit is simple, the cost and the volume of the controller can be greatly reduced, and the current stress flowing into the power switch is greatly reduced due to the change of the current circulation path, so that the power loss in operation is reduced, and the inverter has an industrial application prospect.

Drawings

FIG. 1 is a topology diagram of a three-phase half-bridge-series winding topology switching inverter provided by the present invention;

FIG. 2 is a topology diagram of an inverter in a first mode provided by the present invention;

FIG. 3 is a topology diagram of an inverter in a transient mode provided by the present invention;

FIG. 4 is a topology diagram of an inverter in a second mode provided by the present invention;

FIG. 5 is a three-phase symmetrical AC current provided by the present invention;

FIG. 6 shows the motor operating region of the inverter provided by the present method;

fig. 7 is a state machine diagram of mode switching of the inverter provided by the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

As shown in fig. 1, the present invention provides an inverter with a three-phase half-bridge-series winding topology, which includes a first bridge arm, a second bridge arm, a third bridge arm, a fourth bridge arm, a first triac T1, a second triac T2, a third triac T3, and a fourth triac T4; each bridge arm comprises an upper bridge arm power switch device and a lower bridge arm power switch device, an upper node of the upper bridge arm power switch device of each bridge arm is connected with a direct current bus voltage, a lower node of the lower bridge arm power switch device is connected with a power ground, and a lower node of the upper bridge arm power switch device is connected with an upper node of the lower bridge arm power switch device and serves as an output node of the bridge arm;

the left node of the A-phase winding is connected with the output node of the first bridge arm, and the right node of the A-phase winding is connected with the right node of the first bidirectional thyristor T1 and the left node of the second bidirectional thyristor T2;

the left node of the phase B winding is connected with the output node of the second bridge arm, and the right node of the phase B winding is connected with the right node of the third triac T3 and the left node of the fourth triac T4;

the left node of the C-phase winding is connected with the output node of the third bridge arm, and the right node of the C-phase winding is connected with the output node of the fourth bridge arm;

the left node of the first bidirectional thyristor T1 is connected with the output node of the second bridge arm, the left node of the third bidirectional thyristor T3 is connected with the output node of the third bridge arm, and the right nodes of the second bidirectional thyristor T2 and the fourth bidirectional thyristor T4 are both connected with the output node of the fourth bridge arm.

Further, four triacs T1-T4 are used to switch the topology of the inverter:

when the second bidirectional thyristor T2 and the fourth bidirectional thyristor T4 are switched on and the first bidirectional thyristor T1 and the third bidirectional thyristor T3 are switched off, the three-phase half-bridge topology structure is a three-phase half-bridge topology structure, can provide large current but cannot provide high direct-current voltage utilization rate, and therefore the three-phase half-bridge topology structure is suitable for the motor to operate under the working condition of low speed and high torque;

when the first bidirectional thyristor T1 and the third bidirectional thyristor T3 are switched on and the second bidirectional thyristor T2 and the fourth bidirectional thyristor T4 are switched off, the series winding topological structure can provide high direct-current voltage utilization rate but cannot provide large current, so that the motor is suitable for running under a high-speed working condition, but the torque needs derating running;

when the first triac T1 and the fourth triac T4 are turned on and the second triac T2 and the third triac T3 are turned off, a transient structure is formed.

In particular, the power switch device is a current fully controlled switch, such as a MOSFET or an IGBT with an anti-parallel diode.

The invention also provides a topological structure switching method for the inverter, which aims to smoothly and randomly switch two topological structures, so that the switching process is as short as possible and rapid, and the rotating speed and the torque of the motor are not influenced in the switching process, thereby avoiding influencing the experience of a user. The control method comprises the following steps:

driving the second triac T2 and the fourth triac T4 to be turned on, the first triac T1 and the third triac T3 to be turned off, removing a driving signal of the second triac T2 when receiving a command for switching from the first mode to the second mode, turning off the second triac T2 after waiting for a natural zero-crossing of the a-phase current, driving the first triac T1 to be turned on, and simultaneously removing a driving signal of the fourth triac T4, switching the mode to the transient mode, turning off the fourth triac T4 after waiting for a natural zero-crossing of the B-phase current, driving the third triac T3 to be turned on, and switching the mode to the second mode;

driving the first triac T1 and the third triac T3 to conduct, the second triac T2 and the fourth triac T4 to turn off, when receiving a command for switching from the second mode to the first mode, removing the driving signal of the third triac T3, waiting for the natural zero crossing of the B-phase current, turning off the third triac T3, driving the fourth triac T4 to conduct, and simultaneously removing the driving signal of the first triac T1, switching the mode to the transient mode, waiting for the natural zero crossing of the a-phase current, turning off the first triac T1, driving the second triac T2 to conduct, and switching the mode to the first mode.

Specifically, in the first mode, the inverter has a three-phase half-bridge topology, the second triac T2 and the fourth triac T4 are turned on, and the first triac T1 and the third triac T3 are turned off, as shown in fig. 2; in the transient mode, the inverter is in a transient structure, the first triac T1 and the fourth triac T4 are turned on, and the second triac T2 and the third triac T3 are turned off, as shown in fig. 3; in the second mode, the inverter is in a series winding topology, the first triac T1 and the third triac T3 are turned on, and the second triac T2 and the fourth triac T4 are turned off, as shown in fig. 4.

Fig. 5 shows a three-phase symmetrical ac current waveform of a typical three-phase ac motor. The three-phase current expression corresponding to the current waveform is as follows,

wherein i_a，i_b，i_cPhase currents of phase A, phase B and phase C, I_acThe magnitude of the effective value of the phase current, θ_eIs an electrical angle and is related to the rotor angle.

Specifically, three-phase symmetric alternating current causes different current stresses to the bridge arm power devices in the inverter in different inverter topologies. In the first mode, the current flowing into each arm can be expressed as a stator current of the three-phase alternating-current motor as:

wherein i₁、i₂、i₃And i₄I represents currents flowing in the first arm, the second arm, the third arm, and the fourth arm, respectively_a、i_bAnd i_cRespectively represent the currents of the A-phase stator winding, the B-phase stator winding and the C-phase stator winding, I_acIs the effective value of the AC component of the stator winding current, theta_eIs an electrical angle and is related to the rotor angle. It can be seen that phase currents flow through the first, second and third legs, while no current flows through the fourth leg. In this case, the drive circuit of the fourth arm does not need to operate. Therefore, the current loss of the motor operation is small at the moment, and the motor can operate under the low-speed and high-torque working condition. However, the three-phase half-bridge topology in the first mode has only a dc voltage utilization rate of 57%, and therefore, cannot be applied to a high-speed working condition requiring a high dc voltage utilization rate.

Specifically, in the transient mode, the current flowing into each arm can be represented by the stator current of the three-phase ac motor as:

wherein i₁、i₂、i₃And i₄I represents currents flowing in the first arm, the second arm, the third arm, and the fourth arm, respectively_a、i_bAnd i_cRespectively represent the currents of the A-phase stator winding, the B-phase stator winding and the C-phase stator winding, I_acIs the effective value of the AC component of the stator winding current, theta_eIs an electrical angle and is related to the rotor angle. It can be seen that the current stress for the first leg, third leg, and fourth leg is the phase current, while the current stress for the second leg is 1.717 times the phase current. However, in this transient mode of the inverter, the inverter can still output only half of the utilization rate of the dc voltage, because only 1/3 fundamental wave cycles are operated, without considering the loss problem.

Specifically, in the second mode, the currents flowing into the respective arms can be expressed as the stator currents of the three-phase alternating-current motor as:

wherein i₁、i₂、i₃And i₄I represents currents flowing in the first arm, the second arm, the third arm, and the fourth arm, respectively_a、i_bAnd i_cRespectively represent the currents of the A-phase stator winding, the B-phase stator winding and the C-phase stator winding, I_acIs the effective value of the AC component of the stator winding current, theta_eIs an electrical angle and is related to the rotor angle. It can be seen that the current stress of the first and fourth legs is the phase current, while the current stress of the second and third legs is 1.717 times the phase current. The torque therefore requires derated output. Compared with the first mode or the transient mode, the topological structure in the mode can output twice direct-current voltage utilization rate, so that the high-speed operation interval of the motor is expanded, and flux weakening control is reduced. In addition, since the fourth bridge arm only works during the current derating output, the power device capacity of the bridge arm can be selected to be 0.58 times of the rated current capacity.

Fig. 6 shows the range of operation that can be achieved with an inverter controlled motor based on half-bridge-series topology switching. It can be seen that, by topology switching, the operating interval of the three-phase machine in the first mode is the same as in the three-phase half-bridge topology, and the operating interval in the second mode is the same as in the three-phase series topology. Therefore, the torque output of the motor at low speed is ensured, the high rotating speed range is doubled, and the running range of the motor is greatly expanded.

Another important advantage of inverters based on half-bridge-series topology switching is that the switching process is smooth and fast. During the switching process, the current and the torque do not fluctuate or generate transient processes, so that the influence on a user is avoided. On one hand, the smooth switching process benefits from the fact that when the topological structure is switched, the connection mode of only one phase winding is switched every time, and under the topological structure, the control of three phase windings does not influence each other, so that the smooth switching can be carried out when the phase current in the winding flows through zero; on the other hand, it would also benefit from the use of the control characteristics of the triac. The bidirectional thyristor is controllable in turn-on and uncontrollable in turn-off, and needs to be turned off by natural zero-crossing of current when turned off, so that the driving signal of the bidirectional thyristor can be removed at any time without changing a control method, and the bidirectional thyristor can be turned off by waiting for natural zero-crossing of phase current. The specific mode switching state machine flowchart is shown in fig. 7.

It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.